The separation of gas streams, most notably air into its constituent components, such as oxygen and nitrogen, has been practiced for many years utilizing energy intensive processes for the recovery of various purities and volumes of gas product. Chemical adsorption of individual gas components, fractional cryogenic distillation of various gas components and differential permeation through membrane media have all been practiced in order to recover components of gas streams and more particularly oxygen and/or nitrogen from air. These processes generally suffer from high utility costs or the lack of continuous or convenient regeneration or operation.
Various processes for separation of gas streams, such as air, have been contemplated wherein at least some power is recovered from an effluent stream to be utilized in the power requirements of the process itself.
For instance, in U.S. Pat. No. 4,132,766, a chemical air separation process is set forth wherein air is compressed to elevated pressure before being chemically separated into oxygen and a nitrogen-rich waste stream by the chemical binding of oxygen from the air differentially over the nitrogen. The effluent nitrogen rich stream is then reduced in pressure through an expander before being vented. The expander drives the compressor to recover at least some power for the process. No net power is achieved by the process.
Alternately, it has been suggested, as in European Patent Application No. 0082600, that air may be compressed in a compressor, reduced in temperature by heat exchange against process streams or external cooling means and then separated into oxygen and a nitrogen-rich effluent stream by passage of the compressed and cooled air over a semipermeable membrane which selectively allows the migration of oxygen through the membrane for isolation. The nitrogen-rich effluent stream is then rewarmed by heat exchange against the compressed air feed stream before being expanded to reduced pressure with the recovery of power for the compression requirements of the process. No fuel is combusted and no net power is produced, and in fact, a power import is required in order to overcome inefficiencies in the process, as illustrated in FIG. 1 of the application.
In order to provide additional power for the operation of compression equipment in a gas separation process, it has also been contemplated to combust a waste stream to provide additional power derived from fuel mixed with the waste stream being combusted. For instance, in U.S. Pat. No. 4,340,578 a chemical air separation technique is disclosed wherein air is compressed, separated into oxygen and a nitrogen-rich stream by chemical agents and then the nitrogen rich stream which still contains residual oxygen is mixed with fuel and combusted in a combustion zone wherein the hot effluent of the combustion zone is expanded in several stages through turbines and power is recovered for the compression requirements of the process. No net power is derived.
A similar combustion utilization is described in U.S. Pat. No. 4,224,045 wherein a cryogenic air separation is utilized. In this patent, process air is compressed in a compressor and then reduced significantly in temperature to the liquefaction temperature of the components of the air before being distilled in a cryogenic fractionation column. A waste stream from the low pressure column of the cryogenic separation is rewarmed, recompressed and then combusted with fuel and by-pass air which has not been subject to cryogenic separation, before the combustion effluent is expanded through a turbine to recover power for the process. Net power is derived from the process.
Power generation is contemplated in the cryogenic air separation process described in U.S. Pat. No. 4,382,366. In this process, air is initially compressed and then reduced in temperature to effect a cryogenic separation of oxygen and nitrogen. The nitrogen-rich waste effluent from the cryogenic distillation is rewarmed and combusted with fuel before being expanded through a turbine to lower pressure to power the air compressor to the cryogenic distillation. Additional power is recovered for oxygen product compression, and net electric power can be generated from the expander turbine and an associated steam turbine operating off the waste heat exhausted from the main expander turbine. Although this system produces net power, the overall process for the generation of oxygen is an energy intensive cryogenic or low temperature air distillation scheme.
Additional art of general relevance includes: U.S. Pat. Nos. 2,540,151, 3,713,271, 3,930,814, 4,174,955 and 4,198,213.
The present invention overcomes the disadvantages of the prior art by recovering a minor amount of the oxygen content of air being compressed for power generation with low capital cost and energy requirements for the separation in conjunction with the normal generation of significant amounts of power through combustion and expansion in the turbine equipment of the compressed gases less the oxygen recovered by non-cryogenic separation. This is performed while the air is at elevated temperature.